Object detecting and locating apparatus



G. M. VOGL1S ETAL OBJECT DETECTING AND LOCATING APPARATUS Aug. 24, 19656 Sheets-Sheet 1 Original Filed May 31, 1957 Aug. 24, 1965 G. M. VOGLISETAL OBJECT DETECTING AND LOCATING APPARATUS 6 Sheets-Sheet 2 OriginalFiled May 51, 1957 Aug. 24, 1965 c;. M. VOGL]S ETAL OBJECT DETECTING ANDLOCATING APPARATUS Original Filed May 51, 1957 6 Sheets-Sheet 3 Aug. 24,1965 G. M. VOGLIS ETAL OBJECT DETECTING AND LOCATING APPARATUS 6Sheets-Sheet 4 Original Filed May 31, 1957 6 Sheets-Sheet 5 Aug. 24,1965 G. M. voeus ETAL OBJECI DETECTING AND LOCATING APPARATUS OriginalFiled May 51, 1957 "nom mom

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Aug. 24, 1965 G. M. VOGLIS ETAL OBJECT DETECTING AND LOCATING APPARATUSOriginal Filed May 31, 1957 6 Sheets-Sheec 6 now Now

United States Patent Ofiice 3,22,%l Patented Aug. 24l, 1965 3,202,961BEECT DETECTTNG AND LOCATTNG AP?ARATUS Gregory Michael Voglis,Teddingten, and Hector Ford Willis, St. James Park, London, England,assignors to The Commissioners for Enecnting the 0ffice of Lord HighAdmiral of the United Kingdom of Great Britain and Ireland Originalapplication May 31, 1957, Ser. No. 662,942, new Patent No. 3,161,851,dated Dec. 15, 1964. Divided and this appiication Apr. 23, 1963, Ser.No. 275,832

4 Claims. (Cl. 340-6) This is a division of applicants copendingapplication, Serial No. 662,942, filed May 31, 1957.

This invention concerns apparatus for the rapid detection and locationof objects by directional reception of energy emitted by or refiectedfrom the objects, and, al though the invention is not necessarilyapplicable only to apparatus responsive to acoustic energy, theinvention was primarily developed for the detection and location ofsubmerged objects by the use of acoustic energy, after the manner of thenow well-known Asdic apparatus, so that, for convenience, the followingdescription Will be mainly concerned with the acoustic aspects of theinvention. The Asdic apparatus is described in an article by Baxter,entitled Sound Underwater, in Scientists Against Time, Little, Brown andCo., Boston, 1946.

In the acoustic field the reception of sound and its conversion intoelectrical energy is efiected by a transducer, and the ability of anytransducer to discriminate between sounds from different directions isrepresented by the beamwidth which is defined as propo1tional to theratio )\/D where is the wavelength of the sound in the medium (e.g.liquid) in which the transducer is immersed, and D is the length of thetransducer face.

Ordinary forms of transducer receive eiectively only when the transduceris facing directly towards a sound reflecting or sound emitting object,within an are corresponding to the beamwidth, so that the transducer hasto be turned bodily to search over a larger aro. This way of scanningcannot be used for fast searching because of physical difiiculties andthe limitaton mposed by the time required for sound to travel fromsource to receiver, i.e., to the maximum range and back in the case ofecho reception.

The need for moving the transducer bodily can be avoided by using amultiple element strip transducer and suitably modifying the phaserelationships of the separate outputs from the individual elementsbefore combining them, so as to scan by altering the direction ofmaximum response of the transducer and obtain the electrical equivalentof turning the transducer bodily without actually moving it. The phasechanges required to produce this effect have in previous systems beeneffectcd discontinuously so that the direction of maximum sensitivity ofreception is defiected in steps.

The present invention is an improvement on the aforesaid electricalscanning and enables continuous and rapid deflection of the direction ofbeamed reception to be obtained in a simple manner, so that it isinherently capable of very higr. speed operation. It can be applied toapparatus using a highly directive sound receiver in conjunction with asuitable local transmitter from which sound is emitted over a wide angiewhich is scanncd by the receiver; it may also be applied to the scanningof sounds emanating from distant sources over a wide sector.

The method used has been designated modulation scanning and is based onthe fact that a frequency change imparted to any signal is equivalent toa phase modulation of that signal which is linear with respect to time.More explicitly, suppose that the original signal of frequency f issubjected to a frequency change, the result being a signal of anamplitude proportional to that of the original signal and of frequency+A where Af is the frequency increment. If now this resultant iscompared With its original form, it can be interpreted as a signal atfrequency f the phase of which increases linearly with time, the maximumphase deviation being 27r changing at the rate of 2'1TA rad/sec. As themodulation scanning speed is determined by this rate of phase change, itfollows that it is entirely dependent on the frequency change Af.

The application of this principle to transducers follows simply. If thetransducer consists of N elements and by some means the frequency ofevery signal obtained from the elements is changed so that the resultantfrequencies form the sequences.

where f :f if, f is the acoustic frequency, f a frequency of any valueincluding O, and f the scanning frequency, then the added resultantsrepresent mathemat ically a beam of beamwdth equal to that of the entiretransducer, scanned periodically but continuously over a sector equal toN times the beamwidth at the rate of f scans per sec., the direction orbearing of maximum sensitivity within the scanning sector being at everyin stant uniquely determined by the phase of the scanning cycle, i.e. asharmonic functions of time.

In more general terms, any method of frequency change or modulation,which reduces a set of voltages of the form 7 7L 2 i n 2 27 2 1 n 2; 7L2 for N :2n

to a single function of the type where a and b are constante and N iseither 2n+1 or 2n, can be regarded as a practical realization of thebasic principle of modulation scanning.

The invention accordingly comprises apparatus for directional receptionof sound or supersonic or subsonic energy propagated in a fiuid and forperiodic uniform and continuous deflection of the direction of maximumsensitivity of reception at a selected scanning frequency, comprsing atransducer immersed in the fiuid and sub divided into elements, means ofcoherently modulating the outputs of each element and of processingtheir resultants so as to introduce between the outputs of any elementand its adjacent element to one particular side, eifective and coherentphase shifts linearly varying With time but always the same for alladjacent pairs of elements at every instant of the modulation cycle,means of adding the processed modulation products to produce a singlecarrier modulated electrical signal of carrier frequency smaller orlarger than or equal to the acoustic frequency and of carrier amplitudeproportional to the magnitude of the sound appropriate to the defiectionof the beam at any instant, and means of displaying on a C.R.O. screenthe signal thus derived so as to determine, from the phase of thescanning cycle at the instant of maximum carrier amplitude response, thedirection or bearing of a transmitting or refiecting source of thereceived sound relative to the normal to the transducer face.

Perhaps a better understanding of the inventon can be obtained bycomparison of the signals derived from an analogous mechanically scannedsystem. Imagine for example a horizontal array consisting of an Oddnumber of transducers lying in a vertical reference plane and rotatableabout a vertical axis in the reference plane through the centertransducer. Now imagine that the array is rotating, and has just assumeda position coincident with the reference plane at the instant that aplane wavefront is passing through the reference plane. In this positionthe transducers, with the exception of the center transducer, are movingparallel to the direction of propagation of the wavefront; those to oneside of center moving with the wavefront and those on the opposite sidemoving in the opposite sense. The velocity of each transducer accordingto simple mechanics relative to that of the reference plane isproportional to its distance from the center. The pair of transducersnear est the one at the center, thus, effect a doppler frequency shift Ain the passing wavefront which according to wave mechanics is nearlyproportional to the velocty of that transducer for small changes infrequency. The next nearest transducer being twice as far from thecenter effect a frequency shift 2Af, the third 3A etc. The center beingstationary produces no doppler. Transducers placed between those in theabove array to form a symmetrical even numbered array would obviouslyeffect intermediate values of Af such as /2Af, %Af, etc. The presentinvention produces an apparent rotation by heterodyning the signals froman array with a spectrum of locally generated synchronous signals withfrequences corresponding to the values of Af as set forth above.

Even though the signals at each of the transducers are different in amechanically rotated array there is a certain phase coherence betweensignals. For example, if the entire array were stationary andinstantaneously brought to scan rotation velocity, a cycle of each Afwould start at the same instant. This phase coherence must also bepresent in a heterodyne processing or steering system.

Thus coherence does not require rephasing of the locally generatedsynchronous signals, but means rather that care should be taken to avoidunnecessary shifts in the relative phase of the signals processed.

A more thorough analysis indicates the simulation of one or more,dependngfion the scan frequency, rotating -arrays spaced at angles toone another, but only the one in closest coincidence with the referenceplane of the transducers is eifective. This is due to the limits placedon the input signal available for processing by the directivity of thefixed transducer structure.

The active period of each simulated transducer is one cycle of thelowest Af which may therefore be used as a time base.

The nvention is best understood with reference to the accompanyingdrawings, wherein:

FIGS. 1 and 2 show different circuit embodiments for producing an outputcentered on the acoustic frequency of a scanning system;

FIGS. 3-6 show diflerent circuit embodiments for producing an output ata frequency other than the acoustic frequency of the scanning system ofthe present invention.

- Referring to FIG. 1 a simple form of the invention is shown. As willbe the case with all of the species disclosed heren the array consistsof a plurality of N transducers one of which may be centered and will becalled the zero-th transducer 101. 011 each side of center are ntransducers, the nth transducers 104 and 105 being at the ends of thearray and remaining rth transducers such as 102 and 103 arranged inascending order from the center to each end. The term r is merely anintegral number denoting the order of the transducer and should not beconfused with F which shall be discussed presently.

With the exception of the center transducer the acoustic signalsreceived by each of the transducers becomes an electrical signal and isapplied to the input of a balanced modulator, as for example 110. Eachof these same input signals is also passed through a quarter-wavelengthphase shifter such as 107 and applied to the input of a second balancedmodulator 108. A preamplifier, such as 1ll6, will generally be requiredat each transducer to provide sufficient drive for the modulator.

The phase shifted signals to one side of center are modulated by a localsignal source 109 with a waveform sin w t. The same signals on theopposite side are modulated by a similar signal inverted in phase. Thefactor 7 is a constant which varies directly with the spacing fromcenter and may be simply defined in an equally spaced transducer arrayat the ratio of the distance from center divided by the transducerspacing. The term w is the desired angular scanning frequency of thearray. The remaining non-phase-shifted signals are modulated with asource 111 of cos w t. All of the signals are then applied to a commonadder 113.

Although the resultant signal from the adder would appear to be quitecomplex, an examination of the inputs reveal that this is not so. Theuse of balanced modulators eliminates the original acoustic frequency asan output. The phase shift effected, for example by element 107,together with the phase difference of the modulating signals eliminatesone sideband. The fact that one set of modulating signals is nvertedresults in the elimination of the low frequency sidebands on one sideand the high frequency sidebands on the other. Thus the remainingsignals are just those discussed above in the mechanically rotatedarray.

The resultant adder signal may then be passed through a filter 114 Witha minimum band pass of (Nl)f +df where N is the number of transducers, fthe scanning frequency and df the bandwidth of the transducer; thecenter frequency of the filter being the input acoustic frequency. Thesignal may also be passed through an amplifier 115, if required, todrive a display means. Usually the display means will be a cathode raytube 116, as in the Asdic apparatus mentioned above, and the signaloperates the intensity grid, although it could equally Well be appliedto another control element. A source 118 of scan frequency signal isapplied to one set of deflection plates 117 to provide a time or angularbase for the information displayed.

FIG. 2 shows another embodiment for achieving the results obtained inthe structures described above. Again the transducers 201-205 arearranged as before and each may have a preamplifier 206. The output ofeach preamplifier contains a divider circuit 207 and 208 which producestwo equal signals. To one side of center a half output of each divideris passed through a separate quarter-wave phase shifter 209 and thencedrives a balanced modulator 210. The signal is there modulated by asource 212 of sin w t. Similarly a half output from each transducer onthe opposite side is fed without phase shift to a separate balancedmodulator, for example element 211, and modulated by a source 213 of cosw t. The remaining half-output from each divider is combined inantiphase relationship with the previously mentioned halfoutput of thecomplementary transducer equally spaced from the center of the array onthe opposite side. As shown a transformer type divider is well suitedfor this purpose. The modulator outputs are then combined in an adder214 and may be filtered by element 215, boosted in amplifier 216 anddisplayed by elements 218-219 which correspond to exactly the sameelements found in FIG. 1.

Although it is not immedately apparent the action of this circuit isquite similar to that in the FIG. 1 device. If one considers that asignal is received only by a transducer on one side such as element 202there are two modulator branches for this signal through elements 21)and 211. Similarly, if transducer 203 receives a signal it passesthrough the same two branches. The source of sin w t is inverted on oneside of the array in FIG. l, but this is compensated in the FG. 2 deviceby the antiphase signals supplied by the dividers 207 and 208.

Other embodiments of circuits producing an output centered on theacoustic frequency of the scanning system of the present invention areshown in applicants copending application, Serial No. 662,942, filed May31, 1957.

FIG. 3 shows a scanning system which can be used over a wide range ofacoustic input frequencies. Like the system previously described itcontains a similar group of transducers 3fill-3fi5 each of which mayhave a preamplfier 36. The signals feed a heterodyne processing circuit300 which can be chosen from any of the two preceding species such as1t) in FIG. 1 or 2% in FIG. 2. The acoustic signal from each transducerin this arrangement, however, passes through a difierent balanced tuningmodulator 3t3i and a band-pass filter 309 before it is applied to theprocessing circuit 3%. In element 307 the input signal is modulated by asource 308 of cos wl. The value of w depends on the input acousticfrequency and the intermediate input frequency of the heterodyneprocessing circuit. The value of a: is chosen so that its sum ordifference with the acoustic input frequency equals the intermediatefrequency. Band-pass filter 3i)9 is tuned to the intermediate frequencyand has the same bandwidth as the transducers. With this arrangementonly w needs to be adjusted for a change in acoustic frequency andretuning of the band-pass filters is unnecessary. Elements 313M may bethe same as corresponding structure in preceding embodiments.

The embodiment of FIG. 4 utilizes a similar array of transducers401-4635 each with its own preamplifier 405 feeding a balanced modulator4ti7. The center transducer is modulated by a source 409 of cos cut. Thetransducers on one side of the array are each modulated from a source ofcos (w+w )t, while those on the other side by a source of cos (cuw )t.

This provides the proper components necessary to an apparently tiltingarray each of the components having been translated above and below theacoustic frequency by the factor w. These components are combined in anadder 4211 and passed through filter 412 having a bandpass equal tofilter 114 in FIG. 1. The passband is centered on either the sum ordifference of the acoustic frequency and 0. Again the only adjustmentrequired for changes in the acoustic frequency is a change in w.Elements 413416 correspond to similar elements in preceding embodiments.

FTG. 5 employs the same array 5915fi5 of transducers each with apreamplifier 5% as described in previous embodiments. The output of eachpreamplifier is passed through a divider, as elements 507 and 5il8, andfed to symmetrcally located modulators 51) and 511 as taught in FIG. 2,for example. The modulators on one side of the array are connected to asource of sin wt sin 70: 1, while those on the opposite side are fedfrom a source of cos mt cos Fro t. The center transducer, if present, isprovided with a modulator 569 connected to a source of cos (of.

Comparison with the structure of FIG. 5 with that in FIG. 2 shows thatthe modulation products are similar except that there are two sets inFIG. 5 separated in frequency from those in FIG. 2 by the factor w. Thecomponents are combined in adder 515 and passed through a filter 516with a band-pass similar to filter 114 in FIG.

1. The center frequency of this filter is tuned to either the sum ordifference of the acoustic input frequency and w so that one set ofresultant components is obtained. Elements S1752 may be used to displaythe result as in previous embodiments.

The circuits of the FIG. 6 embodiment are similar in elements 616-611with the exception of modulator 609 provided for the center transducerto those elements in the figures described above. That modulator isconnected to a source 6312 of cos wt. The modulators to the left ofcenter are connected to a source of cos wt sin w z while those to theright are connected to a source of cos wt cos Fw t. The cos wt functionis separated above and below in frequency by the factor w. The signalsare added one to each side, one side is shifted a quarter wave in phaseover the frequency band of filter 618 and added in the circuit 617,thereby canceling undesired compo nents in the spectrum of the outputsignal. Band-pass filter 618 is tuned to have a center frequency at thesum or difference between the acoustic frequency and w and has the samepassband as filter 114 in FIG. 1. Display elements 619-622. are used aspreviously.

The various components for the system heretofore described are all wellknown elements, which have been developed extensively in the prior artand are discussed in standard textbooks on audio and radio engineering.The only problem Which might present itself in circuits of this typewould be that of providing bandwidth for separation and filtering thevarious signals. With the frequency translation systems using sin cutsignals provided the problem of bandwidth is no longer a limitationsince the bandwidth may always be made a small percentage of the signalsin various parts of the system.

The principle of two stage modulation scanning can be readily extendedto any number of stages, for the g outputs of the first modulation stagemay be divided groups of n elements each. Each group is now scannedseparately but coherently at the scanning frequency n .f in a similarmanner as in the first stage of a two scanning process. There Will be gindependent second stage scanned ouputs which can be divided again intogroups and scanned at the frequency n .f This process can be extended toany number of stages without afiecting the final result. For it can beproved mathematically that, provided at any stage, consisting of one ormore groups, the scanning is carried out at a frequency equal to that ofthe proceeding stage times the number of elements contained in each ofits groups, the final output obtained from a multistage scanning processis identical to that derived from a single stage scanning method, theparameters N and f remaining the same.

Multistage modulation scanning shows many advantages over single stagemethods, the most important being a substantial reduction in the numberof modulating voltages required to operate the scanning gear. Thissaving is particularly pronounced when N is a large number i.e. a narrowbeam is to be scanned over a large sector. Tn all such cases multistagemodulation will result in a worth while reduction in the size of thescanning gear as compared with that obtained by application of singlestage modulation scanning methods.

The number of stages to be employed in a multistage scanning process,will largely depend on the magnitude of N but in any case no requirementfor more than 4 stages of modulation are likely to occur in practice.

The multistage structure described above afrords the possibility ofscanning an acoustic beam in two alternative modes of operation.

Consider again. an N element transducer scanned with the frequency f Ifsingle stage modulation is employed the sector scanned is equal toapproximately N times the receiver beam Width and therefore fixed. Onthe other hand, if the beam is scanned by means of multistagemodulation, there are two alternative modes of operation, namely thebeam can be scanned either (1) Over the complete scanning sector at thefrequency fs: or (2) Over 1 of it at the rate of n scans per second,(where n represents the number of elements in the first stagesubtransducer) The changeover is effected by arranging the modulatingvoltages to the first stage modulator so that each first stagesubtransducer output is proportional to the vectorial sum of all n;voltages induced in the elements of one subtransducer. In practice thiscan be accomplished by substituting for the A.C. modulatng voltages tothe modulators of the type cos 7w t of FIGS. 1, 2, 5 and 6 a directcurrent of suitable magnitude and short circuiting the modulationterminals of all remaining modulators.

What we claim is:

l. Apparatus for controllng the directional reception of wave energypropagated in a fluid medium at a selected frequency by an array oftransducers disposed about a central point, comprising:

divider means dividing the signals received from the transducers toeither side of the central point of said array into two components;

modulatng means symmetrically located on each side ofsaid central point,each to receive one signal component from a transducer on one side ofthe array and one from the transducer symmetrically located on the otherside of said array;

said modulatng means modulatng the component signals received from saidtransducers at a fixed frequency;

signal adder means coupled to said modulatng means to combine themodulated signals;

and band-pass filter means for filtering the output from said addermeans.

2. The apparatus as defined in claim l, wherein the modulatng means onone side of said central point modulate the signals from saidtransducers by cos cut cos w t and those on the other side by sin cutsin Fa t, said modulatng means including a modulator for modulatng thesignal from the transducer at the central point of said array by cos mt,where w is a predetermined frequency, w

is the desired annular scanning frequency and F is a constantproportional to transducer spacing.

3. Apparatus as defined in claim l, wherein the modulating means on oneside of said central point modulate the signals from said transducers bycos wt cos w t and those on the other side by cos wt sin Fw t, saidmodulat ing means including a modulator for modulatng the signal fromthe transducer at the central point of said array by cos wz, where w isa predetermined frequency, w is the desired annular scanning frequencyand F is a constant proportional to transducer spacing.

4. Apparatus for controlling the directional reception of wave energypropagated in a fluid medium at a selected frequency by an array oftransducers disposed about a central point, comprising:

a plurality of modulatng means, each coupled to one of said transducersto modulate the signals received therefrom at the same frequency;

band-pass filter means for filtering the modulated signals from each ofsaid transducers,

said band-pass filter means having a passhand substantially equal to thetransducer bandwidth and centered at a frequency representative of thecombination of said selected frequency and said modulation frequency;

a plurality of pairs of modulatng means,

one pair of said pairs of modulatng means being coupled to each of saidband-pass filter means to modulate the signals received therefrom at afrequency proportional to the spacng of each transducer from saidcentral point;

a plurality of phase shift means, each couplng one of said band-passfilter means to one modulatng means from that pair of modulatng meansassociated with said one band-pass filter means;

and combining means to combine signals from said plurality of pairs ofmodulatng means.

References Cited by the Examiner UNITED STATES PATENTS 2,225,928 12/40Ring 343 2,430,296 11/47 Lewis 343- 2,852,772 9/58 Gitzendanner 3431003,012,244 12/61 Langenwalter et al. 343115 3,092,802 6/63 Tucker 3403CHESTER L. JUSTUS, Primary Examiner.

1. APPARATUS FOR CONTROLLING THE DIRECTIONAL RECEPTION OF WAVE ENERGYPROPAGATED IN A FLUID MEDIUM AT A SELECTED FREQUENCY BY AN ARRAY OFTRANDUCERS DISPOSED ABOUT A CENTRAL POINT, COMPRISING: DIVER MEANSDIVIDING THE SIGNALS RECEIVED FROM THE TRANSDUCERS TO EITHER SIDE OF THECENTRAL POINT OF SAID ARRAY INTO TWO COMPONENTS; MODULATION MEANSSYMMETRICALLY LOCATED ON EACH SIDE OF CENTRAL POINT, EACH TO RECEIVE ONESIGNAL COMPONENT FROM A TRANSDUCER ON ONE SIDE OF THE ARRAY AND ON FROMTHE TRANSDUCER SYMMETRICALLY LOCATED ON THE OTHER SIDE OF SAID ARRAY;SAID MODULATING MEANS MODULATING THE COMPONENT SIGNALS RECEIVED FROMSAID TRANSDUCERS AT A FIXED FREQUENCY; SIGNAL ADDER MEANS COUPLED TOSAID MODULATING MEANS TO COMBINE THE MODULATED SIGNALS; AND BAND-PASSFILTER MEANS FOR FILTERING THE OUTPUT FROM SAID ADDER MEANS.